Bronchoalveolar lavage of healthy human adults after LPS exposure

Rationale: Macrophages are both drivers of dysregulated inflammation and essential for healthy repair following acute lung injury. Distinct subsets of human macrophages have been associated with lung pathology but whether unique cellular programs are causally linked to disease or merely represent a conserved response to tissue damage is unknown. Objectives: We sought to identify the transcriptional heterogeneity and cellular programing of airspace macrophages (AM) during normal lung repair in a human model of self-resolving acute lung injury. Methods: Fifteen subjects underwent bronchoscopic lavage before and at a pre-assigned time point after endobronchial exposure to bacterial endotoxin. Cells from lavage were subjected to single-cell RNA sequencing and a longitudinal assessment of AM programing during resolution of inflammation and lung repair was performed. Measurements and Main Results: We identify transcriptionally distinct subsets of AM present at all time points, in all subjects. We focus on two populations that increase following inflammation, one which aligns closely with classical circulating monocytes (MoAM) and one with interstitial macrophages (IMAM). Comparison of subset-specific markers to those identified in disease states reveals that IMAM express many so-called “pathogenic” macrophage markers during both health and inflammation. Conclusions: By applying a uniform inflammatory stimulus to healthy adults and examining BAL cells obtained at precise time points following inflammation, we construct a time-resolved kinetic of AM transcriptional programing during inflammation and normal lung repair. Our data suggest that IMAM are similar to cells identified at increased frequency in disease states and may reflect a conserved cellular response to tissue injury.